res_rtp_asterisk: Fix reported packet loss to use current report's difference

The lost packet calculation was using the cumulative number of lost packets instead
of the current interval's lost packets.  This lead to the MES caculation skewing
over time.  We now keep track of the last reported value so we can determine the
current value vs the cumulative value.

Fixes: #2101

res/res_rtp_asterisk.c

@@ -559,6 +559,7 @@ struct ast_rtcp {
 	double rtt;			/*!< Last reported rtt */
 	double reported_jitter;	/*!< The contents of their last jitter entry in the RR in seconds */
 	unsigned int reported_lost;	/*!< Reported lost packets in their RR */
+	unsigned int last_reported_lost; /*!< Reported cumulative lost packets in the previous RR */
 
 	double reported_maxjitter; /*!< Maximum reported interarrival jitter */
 	double reported_minjitter; /*!< Minimum reported interarrival jitter */
@@ -6398,7 +6399,20 @@ static void update_lost_stats(struct ast_rtp *rtp, unsigned int lost_packets)
 	double reported_lost;
 
 	rtp->rtcp->reported_lost = lost_packets;
-	reported_lost = (double)rtp->rtcp->reported_lost;
+
+	/*
+	 * lost_packets contains the cumulative number of lost packets as reported in
+	 * the peer's RTCP RR/SR report block (RFC 3550). Calculate the number of lost
+	 * packets in the current interval based on the difference from the previous
+	 * count.
+	 */
+	reported_lost = (double)lost_packets - (double)rtp->rtcp->last_reported_lost;
+	rtp->rtcp->last_reported_lost = lost_packets;
+
+	if (reported_lost < 0) {
+		reported_lost = 0;
+	}
+
 	if (rtp->rtcp->reported_lost_count == 0) {
 		rtp->rtcp->reported_minlost = reported_lost;
 	}